1. Technical Field
The present disclosure relates to a technique for implementing a bidirectional DC-DC converter applied to an energy storage system, and more particularly, to a bidirectional DC-DC converter capable of achieving a high gain through a two-step voltage transformation process when a charging or discharging process is performed between a DC link power supply and a battery power supply.
2. Related Art
An ESS (Energy Storage System) includes a PCS (Power Conversion System), a BMS (Battery Management System), and an EMS (Energy Management System) for controlling the ESS. The PCS serves to convert power supplied from various energy sources into a common AC voltage or a voltage suitable for being stored in a battery cell. At this time, energy conversion is required in both directions between the battery cell and the voltage of a DC link. Such a role is performed by PCS which is referred to as a bidirectional DC-DC converter.
In the case of a battery cell, a model for providing various types of voltages has been proposed. However, since a voltage and capacity provided from a unit battery cell are low, a large number of battery cells are connected in series/parallel in order to cope with a load. If a bidirectional DC-DC converter having a high gain between the DC link and the battery can be designed, the number of battery cells connected in series can be reduced to secure a competitive price, and an additional cost used for a battery cell management system can be reduced.
FIG. 1 is a circuit diagram of a conventional bidirectional buck booster-type DC-DC converter. As illustrated in FIG. 1, the bidirectional buck booster-type DC/DC converter includes a DC link VDC, switches Q11 and Q12, an inductor L11 and a battery cell module (battery pack) 11. The switches Q11 and Q12 may include MOS transistors, and the battery cell module 11 may include battery cells coupled in series and parallel.
Referring to FIG. 1, a pair of switches Q11 and Q12 are complementarily operated in a charge/discharge mode. Thus, the battery cell module 11 is charged with power of the DC link VDC through the inductor L11, or discharged through the DC link VDC.
The conventional bidirectional buck booster-type DC/DC converter has advantages in that the basic structure thereof is simple and the charging/discharging control structure for the battery cell module is simple. However, the conventional bidirectional buck booster-type DC/DC converter requires a large number of battery cells connected in series to the battery cell module, because the voltage conversion ratio is low.
FIG. 2 is a circuit diagram of a conventional bidirectional flyback-type DC-DC converter. As illustrated in FIG. 2, the conventional bidirectional flyback-type DC-DC converter includes switches Q21 and Q22, inductors L21 to L23, a transformer TR21 and a battery cell module 21. The switches Q21 and Q22 include MOS transistors, and the battery cell module 21 includes battery cells connected in series and parallel.
Referring to FIG. 2, the pair of switches Q21 and Q22 are complementarily operated in a charge/discharge mode. Thus, the battery cell module 21 is charged with power of the DC link VDC through the inductors L21 to L23, or discharged through the DC link VDC.
The conventional DC-DC converters can insulate the DC link and the battery cell module from each other through the transformer, and control a voltage gain by adjusting the turn ratio of the transformer. However, due to a voltage spike caused by leakage inductance of the transformer, the stability of the converters may be degraded.
FIG. 3 is a circuit diagram of a conventional bidirectional buck boost-type DC-DC converter. As illustrated in FIG. 3, the conventional bidirectional buck boost-type DC-DC converter includes switches Q31 and Q32, inductors L31 and L23 and a battery cell module 31.
The converter of FIG. 3 has an improved architecture, compared to the bidirectional buck booster-type DC-DC converter of FIG. 1.
The conventional DC-DC converters can implement a high voltage conversion ratio using the magnetically coupled inductors. However, due to a sudden change of leakage inductance current, a high voltage spike may be caused to degrade stability. Furthermore, since the conventional DC-DC converters have a low available range, using the conventional DC-DC converters has many limitations.